Basic: Add support for -swift-version 7 to asserts mode compilers.
Begin accepting `-swift-version 7` in asserts compilers so that we can begin to
test source-breaking changes intended for the next language version mode, such
as diagnostics that need to be staged in without breaking existing code. There
are no announced plans yet for a language mode after Swift 6, so 7 serves as a
placeholder for some future language mode.
Tests that wish to use `-swift-version 7` currently must be marked with
`REQUIRES: swift7` or they will fail in no-asserts bots. The simplest pattern
is probably to duplicate part or all of the pre-Swift 7 test to check that will
behave as intended in Swift 7.
swift
47454e3f - Basic: Add support for -swift-version 7 to asserts mode compilers.
